1. Field of the Invention
The present invention relates generally to a steam turbine `different material` welded rotor, and more specifically to one having different materials of a high temperature portion and a low temperature portion jointed together by welding with an increased joint strength and facilitating inspection of defects in the rotor.
2. Description of the Prior Art
In a recent steam turbine rotor, following a tendency to high temperatures of steam, a 12% chromium steel is used, as it is excellent in high temperature strength and toughness. In such a rotor, both for a high temperature portion exposed to a high temperature steam and a low temperature portion exposed to a low temperature steam, the same 12% chromium steel is used. But as the rotor has become large-sized in recent years, it is becoming difficult to manufacture the rotor so as to satisfy characteristics both of the high temperature portion and the low temperature portion with one material. Also, it is practice to use the 12% chromium steel for manufacture of the rotor portion and to use a low alloy steel for sleeves around the rotor, couplings and the like.
While the 12% chromium steel, which is an expensive material, satisfies heat resistance, creep characteristic, etc. of the portion exposed to the high temperature steam, it is not necessary to use such an expensive material for the low temperature portion, but rather toughness is required therefor, and a low temperature material may well respond thereto. In order to meet these problems, as shown in FIG. 7 for example, it is attempted to joint rotor portions of different materials together by welding to make a single rotor.
In FIG. 7, numeral 31 designates a turbine rotor of the 12% chromium steel to be applied to the high temperature portion, and numeral 32 designates a turbine rotor of a low alloy steel to be applied to the low temperature portion, the low alloy steel being an alloy steel containing chromium, molybdenum, vanadium, nickel, etc. Numeral 33 designates a clad portion, which is made of a 9Cr steel and is provided to an end portion 34 of the turbine rotor 31. Numeral 35 designates a groove, which is formed between the clad portion 33 and the turbine rotor 32 to be jointed together by welding via a weld metal 36 to constitute a single rotor.
As described above, the steam turbine rotor, extending from the high temperature portion to the low temperature portion, comprises a portion exposed to a high temperature high pressure steam and a portion exposed to a low temperature low pressure steam. This steam turbine rotor has so far been made in one unit by a single material of the 12% chromium steel, but the 12% chromium steel is expensive, and for this reason, an attempt is being made to joint the rotor portions of different materials to make one rotor, as shown in FIG. 7. But, in such a steam turbine rotor in the prior art as having different materials jointed together, there is a problem in that the weld strength differs according to a position in the joint portion, and moreover, there is paid no attention to a rotor structure to enable a sufficient post-weld inspection of defects.